Separation of ortho bi-alkyl substituted monocyclic aromatic isomers

ABSTRACT

A process for separating the ortho-isomer from a feed mixture comprising at least two bi-alkyl substituted monocyclic aromatic isomers, including the ortho-isomers, the isomers having from 9 to about 18 carbon atoms per molecule. The process comprises contacting, at adsorption conditions, the feed with an adsorbent comprising a crystalline aluminosilicate consisting essentially of a sodium or calcium Y-type zeolite or sodium X-type zeolite which selectively adsorbs the ortho-isomers. The feed is removed from the adsorbent, and the ortho-isomers recovered by desorption at desorption conditions with a desorbent material comprising a C 8  or heavier monocyclic aromatic with a boiling point at least 5° C. different than that of the feed mixture when the adsorbent is the sodium Y-type zeolite, or a C 8  or lighter monocyclic aromatic when the adsorbent is a sodium X or calcium Y-type zeolite. 
     The process is particularly amenable to the employment of a simulated moving bed flow scheme.

BACKGROUND OF THE INVENTION

The field of art to which the claimed invention pertains is hydrocarbonseparation. More specifically, the invention relates to a process forseparating an ortho-isomer from a feed mixture comprising at least twobi-alkyl substituted monocyclic aromatic isomers, including theortho-isomer, which process employs a particular zeolitic adsorbent.

BACKGROUND INFORMATION

It is well known in the separation art that certain crystallinealuminosilicates can be used to separate one hydrocarbon type fromanother hydrocarbon type. The separation of normal paraffins frombranched chain paraffins, for example, can be accomplished by using atype A zeolite which has pore openings from 3 to about 5 Angstroms. Sucha separation process is disclosed in U.S. Pat. Nos. 2,985,589 toBroughton et al. and 3,201,491 to Stine. These adsorbents allow aseparation based on the physical size differences in the molecules byallowing the smaller or normal hydrocarbons to be passed into thecavities within the zeolitic adsorbent, while excluding the larger orbranched chain molecules.

In addition to being used in processes for separating hydrocarbon types,adsorbents comprising type X or Y zeolites have also been employed inprocesses to separate individual hydrocarbon isomers. In the processesdescribed, for example, in U.S. Pat. Nos. 3,626,020 to Neuzil, 3,663,638to Neuzil, 3,655,046 to de Rosset, 3,668,266 to Chen et al., 3,686,343to Bearden Jr. et al., 3,700,744 to Berger et al., 3,734,974 to Neuzil,3,894,109 to Rosback, 3,997,620 to Neuzil and B426,274 to Hedge,particular zeolitic adsorbents are used to separate the para isomer ofbi-alkyl substituted monocyclic aromatics from the other isomers,particularly para-xylene from other xylene isomers. It is also knownthat the adsorptive capacity of certain zeolites for certain separationsis improved by contacting the zeolite with an aqueous caustic solution.In U.S. Pat. No. 3,374,182 to Young, for example, zeolites so treatedare said to be effective in separating aromatic hydrocarbons fromnon-aromatic hydrocarbons of the same molecular size.

In U.S. Pat. No. 4,376,226 to Rosenfeld et al. the separation of theortho aromatic isomer from a mixture of aromatic isomers is disclosedusing CSZ-1 as a crystalline aluminosilicate adsorbent and amon-aromatic hydrocarbon as a desorbent material. CSZ-1 has acomposition in terms of mole ratios of oxides of 0.05 to 0.55 cesiumand/or thallium: 0.45 to 0.95 Na₂ O:Al₂ O₃ :3 to 7 SiO₂ :XH₂ O, where Xis 0 to 10. The CSZ-1 must contain cesium and/or thallium. Themono-aromatic desorbents specifically mentioned are toluene, benzene anddiethylbenzene.

SUMMARY OF THE INVENTION

In brief summary the present invention is, in one embodiment, a processfor separating the ortho-isomer from a feed mixture comprising at leasttwo bi-alkyl substituted monocyclic aromatic isomers, including theortho-isomer, the isomers having from 9 to about 18 carbon atoms permolecule. The process comprises contacting, at adsorption conditions,the feed with an adsorbent comprising a crystalline aluminosilicateconsisting essentially of a sodium or calcium Y-type zeolite or sodiumX-type zeolite which selectively adsorbs the ortho-isomer. The feed isremoved from the adsorbent, and the ortho-isomer then recovered bydesorption at desorption conditions with a desorbent material comprisinga C₈ or heavier monocyclic aromatic with a boiling point at least 5° C.different than that of the feed mixture when the adsorbent is the sodiumY-type zeolite, or a C₈ or lighter monocyclic aromatic when theadsorbent is a sodium X or calcium Y-type zeolite.

Other embodiments of the present invention encompass details about feedmixtures, flow schemes and operating conditions, all of which arehereinafter disclosed in the following discussion of each of the facetsof the present invention.

DESCRIPTION OF THE INVENTION

At the outset the definitions of various terms used throughout thisspecification will be useful in making clear the operation, objects andadvantages of the present invention.

A "feed mixture" is a mixture containing one or more extract componentsand one or more raffinate components to be fed to an adsorbent of ourprocess. The term "feed stream" indicates a stream of feed mixture whichpasses to an adsorbent used in the process.

An "extract component" is a type of compound or a compound, such as anaromatic isomer, that is more selectively adsorbed by the adsorbentwhile a "raffinate component" is a compound or type of compound that isless selectively adsorbed. In this process, the ortho-isomer is theextract component and one or more other aromatic isomers is a raffinatecomponent. The term "raffinate stream" or "raffinate output stream"means a stream through which a raffinate component is removed from anadsorbent. The composition of the raffinate stream can vary fromessentially 100% desorbent material (hereinafter defined) to essentially100% raffinate components. The term "extract stream" or "extract outputstream" shall mean a stream through which an extract material which hasbeen desorbed by a desorbent material is removed from the adsorbent. Thecomposition of the extract stream, likewise, can vary from essentially100% desorbent material to essentially 100% extract components. Althoughit is possible by the process of this invention to produce high-purityextract product (hereinafter defined) or a raffinate product(hereinafter defined) at high recoveries, it will be appreciated that anextract component is never completely adsorbed by the adsorbent, nor isa raffinate component completely non-adsorbed by the adsorbent.Therefore, small amounts of a raffinate component can appear in theextract stream, and, likewise, small amounts of an extract component canappear in the raffinate stream. The extract and raffinate streams thenare further distinguished from each other and from the feed mixture bythe ratio of the concentrations of an extract component and a specificraffinate component, both appearing in the particular stream. Forexample, the ratio of concentration of the more selectively adsorbedortho-isomer to the concentration of less selectively adsorbed para- ormeta-isomer will be highest in the extract stream, next highest in thefeed mixture, and lowest in the raffinate stream. Likewise, the ratio ofthe less selectively adsorbed meta- or para-isomers to the moreselectively adsorbed ortho-isomer will be highest in the raffinatestream, next highest in the feed mixture, and the lowest in the extractstream. The term "desorbent mixture" shall mean generally a materialcapable of desorbing an extract component. The term "desorbent stream"or "desorbent input stream" indicates the stream through which desorbentmaterial passes to the adsorbent. When the extract stream and theraffinate stream contain desorbent materials, at least a portion of theextract stream and preferably at least a portion of the raffinate streamfrom the adsorbent will be passed to separation means, typicallyfractionators, where at least a portion of desorbent material will beseparated at separation conditions to produce an extract product and araffinate product. The terms "extract product" and "raffinate product"mean products produced by the process containing, respectively, anextract component and a raffinate component in higher concentrationsthan those found in the respective extract stream and the raffinatestream. The term "selective pore volume" of the adsorbent is defined asthe volume of the adsorbent which selectively adsorbs extract componentsfrom a feed mixture. The term "non-selective void volume" of anadsorbent is the volume of an adsorbent which does not selectivelyretain an extract component from a feed mixture. This volume includesthe cavities of the adsorbent which contain no adsorptive sites and theinterstitial void spaces between adsorbent particles. The selective porevolume and the non-selective void volume are generally expressed involumetric quantities and are of importance in determining the properflow rates of fluid required to be passed into the process for efficientoperations to take place for a given quantity of adsorbent.

Feed mixtures which can be utilized in the process of this inventionwill comprise at least two bi-alkyl substituted monocyclic aromaticisomers. These isomers can be characterized by reference to Formula 1below: ##STR1## wherein R₁, R₂, R₃ and R₄ are selected from the group ofalkyl chains and positioned in a manner to provide bi-alkyl substitutionat either ortho-, meta-, or para-isomer positions. The R substitutionalgroups can include alkyl groups ranging from methyl substitution groupsup to and including chains having 11 or less carbon atoms per chain. Thealkyl side chains can be both normal and branched in nature and arepreferably saturated chains.

Thus feed mixtures to this process can contain such specificrepresentative compounds as the various isomers of methylethylbenzene,diethylbenzene, isopropyltoluene (cymene), the methylpropylbenzenes,ethylpropylbenzenes, methylbutylbenzenes, ethylbutylbenzenes,dipropylbenzenes, methylpentylbenzenes, etc., and combinations thereof.The above list only represents a small fraction of compounds whoseisomers can be separated by the adsorptive-separation process of thisinvention. Thus the process of this invention will be used for exampleto separate ortho-methylethylbenzene from a feed mixture comprisingortho-methylethylbenzene and at least one other methylethylbenzeneisomer; ortho-diethylbenzene from a feed mixture comprisingortho-diethylbenzene and at least one other diethylbenzene isomer; andortho-cymene from a feed mixture comprising ortho-cymene and at leastone other cymene isomer to name a few. The most likely separations foremployment of the present invention, however, is the separation ofortho-ethyltoluene from other ethyltoluene isomers orortho-diethylbenzene from other diethylbenzene isomers.

The isomers of such compounds are separated by this adsorbent accordingto their configuration depending whether they are of a para-, meta-, orortho-isomer construction. Specifically, the ortho-isomer is selectivelyadsorbed relative to the other isomers. It is contemplated that whenfeed mixtures contain more than one homolog of isomers (for example, C₉isomers in mixture with C₁₀ or C₁₁ isomers) molecular weight differencesmay unduly interfere with selective adsorption based upon isomerconfiguration differences. It is therefore preferred that feed mixturesto be separated by this process contain only a single class of aromaticisomers, that is, aromatic isomers having an equal number of carbonatoms per molecule. It is preferable that the isomers have as their onlydifferences the location of the alkyl substituted groups in a para-,meta-, or ortho-position. The alkyl structures should preferably be thesame for each isomer of a class. In some instances an isomer may havealkyl chains which are both normal or branched or one branched and onenormal.

The feed mixtures may contain small quantities of straight or branchedchain paraffins, cycloparaffins, or olefinic material. It is preferableto have these quantities at a minimum amount in order to preventcontamination of products from this process by materials which are notselectively adsorbed or separated by the adsorbent. Preferably theabove-mentioned contaminants should be less than about 20% of the volumeof the feed mixture passed into the process.

To separate the ortho-isomer from a feed mixture containing ortho-isomerand at least one other aromatic isomer, the mixture is contacted withthe adsorbent and the ortho-isomer is more selectively adsorbed andretained by the adsorbent while the other isomers are relativelyunadsorbed and are removed from the interstitial void spaces between theparticles of adsorbent and the surface of the adsorbent. The adsorbentcontaining the more selectively adsorbed ortho-isomer is referred to asa "rich" adsorbent--rich in the more selectively adsorbed ortho-isomer.The ortho-isomer is then recovered from the rich adsorbent by contactingthe rich adsorbent with a desorbent material.

The term "desorbent material" as used herein shall mean any fluidsubstance capable of removing a selectively adsorbed feed component fromthe adsorbent. Generally, in a swing-bed system in which the selectivelyadsorbed feed component is removed from the adsorbent by a purge stream,desorbent material selection is not too critical and desorbent materialscomprising gaseous hydrocarbons such as methane, ethane, etc., or othertypes of gases such as nitrogen or hydrogen may be used at elevatedtemperatures or reduced pressures or both to effectively purge theadsorbed feed component from the adsorbent. However, in adsorptiveseparation processes which employ zeolitic adsorbents and which aregenerally operated continuously at substantially constant pressures andtemperatures to insure liquid phase, the desorbent material relied uponmust be judiciously selected to satisfy several criteria. First, thedesorbent material must displace the extract components from theadsorbent with reasonable mass flow rates without itself being sostrongly adsorbed as to unduly prevent the extract component fromdisplacing the desorbent material in a following adsorption cycle.Expressed in terms of the selectivity (hereinafter discussed in moredetail), it is preferred that the adsorbent be more selective for theextract component with respect to a raffinate component than it is forthe desorbent material with respect to a raffinate component. Secondly,desorbent materials must be compatible with the particular adsorbent andthe particular feed mixture. More specifically, they must not reduce ordestroy the critical selectivity of the adsorbent for the extractcomponents with respect to the raffinate component. Desorbent materialsto be used in the process of this invention should additionally besubstances which are easily separable from the feed mixture that ispassed into the process. After desorbing the extract components of thefeed, both desorbent material and the extract components are typicallyremoved in admixture from the adsorbent. Likewise, one or more raffinatecomponents is typically withdrawn from the adsorbent in admixture withdesorbent material and without a method of separating at least a portionof desorbent material, such as distillation, neither the purity of theextract product nor the purity of the raffinate product would be veryhigh. It is therefore contemplated that any desorbent material used inthis process will have a substantially different average boiling pointthan that of the feed mixture to allow separation of desorbent materialfrom feed components in the extract and raffinate streams by simplefractionation thereby permitting reuse of desorbent material in theprocess. The term "substantially different" as used herein shall meanthat the difference between the average boiling points between thedesorbent material and the feed mixture shall be at least about 5° C.The boiling range of the desorbent material may be higher or lower thanthat of the feed mixture.

In the preferred isothermal, isobaric, liquid-phase operation of theprocess of this invention, it has been found that the effectivedesorbent materials comprise the C₈ or heavier monocyclic aromatics whenthe adsorbent is a Y-type zeolite, or a C₈ or lighter monocyclicaromatic when the adsorbent is a sodium X or calcium Y-type zeolite.This is in contradistinction to the aforementioned U.S. Pat. No.4,376,226 to Rosenfeld et al. which specifically discloses toluene,benzene and diethylbenzene for use as desorbents with the same zeoliticadsorbent.

The prior art has recognized that certain characteristics of adsorbentsare highly desirable, if not absolutely necessary, to the successfuloperation of a selective adsorption process. Among such characteristicsare: adsorptive capacity for some volume of an extract component pervolume of adsorbent; the selective adsorption of an extract componentwith respect to a raffinate component and the desorbent material; and,sufficiently fast rates of adsorption and desorption of the extractcomponents to and from the adsorbent.

Capacity of the adsorbent for adsorbing a specific volume of one or moreextract component is, of course, a necessity; without such capacity theadsorbent is useless for adsorptive separation. Furthermore, the higherthe adsorbent's capacity for an extract component the better is theadsorbent. Increased capacity of a particular adsorbent makes itpossible to reduce the amount of adsorbent needed to separate theextract component contained in a particular charge rate of feed mixture.A reduction in the amount of adsorbent required for a specificadsorptive separation reduces the cost of the separation process. It isimportant that the good initial capacity of the adsorbent be maintainedduring actual use in the separation process over some economicallydesirable life.

The second necessary adsorbent characteristic is the ability of theadsorbent to separate components of the feed; or, in other words, thatthe adsorbent possess adsorptive selectivity, (B), for one component ascompared to another component. Relative selectivity can be expressed notonly for one feed component as compared to another but can also beexpressed between any feed mixture component and the desorbent material.The selectivity, (B), as used throughout this specification is definedas the ratio of the two components of the adsorbed phase over the ratioof the same two components in the unadsorbed phase at equilibriumconditions.

Relative selectivity is shown as Equation 1 below: ##EQU1## where C andD are two components of the feed represented in volume percent and thesubscripts A and U represent the adsorbed and unadsorbed phasesrespectively. The equilibrium conditions are determined when the feedpassing over a bed of adsorbent does not change composition aftercontacting the bed of adsorbent. In other words, there is no nettransfer of material occurring between the unadsorbed and adsorbedphases.

Where selectivity of two components approaches 1.0 there is nopreferential adsorption of one component by the adsorbent with respectto the other; they are both adsorbed (or non-adsorbed) to about the samedegree with respect to each other. As the (B) becomes less than orgreater than 1.0 there is a preferential adsorption by the adsorbent forone component with respect to the other. When comparing the selectivityby the adsorbent of one component C over component D, a (B) larger than1.0 indicates preferential adsorption of component C within theadsorbent. A (B) less than 1.0 would indicate that component D ispreferentially adsorbed leaving an unadsorbed phase richer in componentC and an adsorbed phase richer in component D. While separation of anextract component from a raffinate component is theoretically possiblewhen the selectivity of the adsorbent for the extract component withrespect to the raffinate component just exceeds a value of 1.0, it ispreferred that such selectivity have a value approaching or exceeding 2.Like relative volatility, the higher the selectivity the easier theseparation is to perform. Higher selectivities permit a smaller amountof adsorbent to be used in the process. Ideally desorbent materialsshould have a selectivity equal to about 1 or less than 1 with respectto all extract components so that all of the extract components can beextracted as a class and all raffinate components clearly rejected intothe raffinate stream.

The third important characteristic is the rate of exchange of theextract component of the feed mixture material or, in other words, therelative rate of desorption of the extract component. Thischaracteristic relates directly to the amount of desorbent material thatmust be employed in the process to recover the extract component fromthe adsorbent; faster rates of exchange reduce the amount of desorbentmaterial needed to remove the extract component and therefore permit areduction in the operating cost of the process. With faster rates ofexchange, less desorbent material has to be pumped through the processand separated from the extract stream for reuse in the process.

In order to test various adsorbents and desorbent material with aparticular feed mixture to measure the adsorbent characteristics ofadsorptive capacity and selectivity and exchange rate, a dynamic testingapparatus is employed. The apparatus consists of an adsorbent chamber ofapproximately 70 cc volume having inlet and outlet portions at oppositeends of the chamber. The chamber is contained within a temperaturecontrol means and, in addition, pressure control equipment is used tooperate the chamber at a constant predetermined pressure.Chromatographic analysis equipment can be attached to the outlet line ofthe chamber and used to analyze "on-stream" the effluent stream leavingthe adsorbent chamber.

A pulse test, performed using this apparatus and the following generalprocedure, is used to determine selectivities and other data for variousadsorbent systems. The adsorbent is filled to equilibrium with aparticular desorbent by passing the desorbent material through theadsorbent chamber. At a convenient time, a pulse of feed containingknown concentrations of a non-adsorbed paraffinic tracer (n-nonane forinstance) and of the particular aromatic isomers all diluted indesorbent is injected for a duration of several minutes. Desorbent flowis resumed, and the tracer and the aromatic isomers are eluted as in aliquid-solid chromatographic operation. The effluent can be analyzed byon-stream chromatographic equipment and traces of the envelopes ofcorresponding component peaks developed. Alternately, effluent samplescan be collected periodically and later analyzed separately by gaschromatography.

From information derived from the chromatographic traces, adsorbentperformance can be rated in terms of capacity index for an extractcomponent, selectivity for one isomer with respect to the other, and therate of desorption of an extract component by the desorbent. Thecapacity index may be characterized by the distance between the centerof the peak envelope of the selectively adsorbed isomer and the peakenvelope of the tracer component or some other known reference point. Itis expressed in terms of the volume in cubic centimeters of desorbentpumped during this time interval. Selectivity, (B), for an extractcomponent with respect to a raffinate component may be characterized bythe ratio of the distance between the center of an extract componentpeak envelope and the tracer peak envelope (or other reference point) tothe corresponding distance between the center of a raffinate componentpeak envelope and the tracer peak envelope. The rate of exchange of anextract component with the desorbent can generally be characterized bythe width of the peak envelopes at half intensity. The narrower the peakwidth the faster the desorption rate. The desorption rate can also becharacterized by the distance between the center of the tracer peakenvelope and the disappearance of an extract component which has justbeen desorbed. This distance is again the volume of desorbent pumpedduring this time interval.

The adsorbent to be used in the process of this invention comprises aspecific crystalline aluminosilicate. Crystalline aluminosilicates suchas that encompassed by the present invention include crystallinealuminosilicate cage structures in which the alumina and silicatetrahedra are intimately connected in an open three-dimensionalnetwork. The tetrahedra are cross-linked by the sharing of oxygen atomswith spaces between the tetrahedra occupied by water molecules prior topartial or total dehydration of this zeolite. The dehydration of thezeolite results in crystals interlaced with cells having moleculardimensions. Thus, the crystalline aluminosilicates are often referred toas "molecular sieves" when the separation which they effect is dependentessentially upon differences between the sizes of the feed molecules as,for instance, when smaller normal paraffin molecules are separated fromlarger isoparaffin molecules by using a particular molecular sieve. Inthe process of this invention, however, the term "molecular sieves"although widely used is not strictly suitable since the separation ofspecific aromatic isomers is apparently dependent on differences inelectrochemical attraction of the different isomers and the adsorbentrather than on pure physical size differences in the isomer molecules.

In hydrated form, the crystalline aluminosilicates generally encompassthose zeolites represented by the Formula below:

    M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O       Formula 1

where "M" is a cation which balances the electrovalence of thetetrahedra and is generally referred to as an exchangeable cationicsite, "n" represents the valence of the cation, "w" represents the molesof SiO₂, and "y" represents the moles of water. The generalized cation"M" may be monovalent, divalent or trivalent cations or mixturesthereof.

The prior art has generally recognized that adsorbents comprising thetype X and the type Y zeolites can be used in certain adsorptiveseparation processes. These zeolites are well known to the art.

The type X structured zeolite in the hydrated or partially hydrated formcan be represented in terms of mole oxides as shown in Formula 2 below:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.5±0.5)SiO.sub.2 :yH.sub.2 O Formula 2

where "M" represents at least one cation having a valence of not morethan 3, "n" represents the valence of "M", and "y" is a value up toabout 9 depending upon the identity of "M" and the degree of hydrationof the crystal. As noted from Formula 2 the SiO₂ /Al₂ O₃ mole ratio is2.5±0.5. The cation "M" may be one or more of a number of cations suchas the hydrogen cation, the alkali metal cation, or the alkaline earthcations, or other selected cations, and is generally referred to as anexchangeable cationic site. As the type X zeolite is initially prepared,the cation "M" is usually predominately sodium and the zeolite istherefore referred to as a sodium-type X zeolite. Depending upon thepurity of the reactants used to make the zeolite, other cationsmentioned above may be present, however, as impurities.

The type Y structured zeolite in the hydrated or partially hydrated formcan be similarly represented in terms of mole oxides as in Formula 3below:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O Formula 3

where "M" is at least one cation having a valence not more than 3, "n"represents the valence of "M", "w" is a value greater than about 3 up to6, and "y" is a value up to about 9 depending upon the identity of "M",and the degree of hydration of the crystal. The SiO₂ /Al₂ O₃ mole ratiofor type Y structured zeolites can thus be from about 3 to about 6. Likethe type X structured zeolite, the cation "M" may be one or more of avariety of cations but, as the type Y zeolite is initially prepared, thecation "M" is also usually predominately sodium. The type Y zeolitecontaining predominately sodium cations at the exchangeable cationicsites is therefore referred to as a sodium type-Y zeolite. It is onlysodium or calcium type-Y or sodium type-X zeolites that may be used inthe process of the present invention without amounts of other cationswhich would materially affect the characteristics of the adsorbent. Thisis in contradistinction to Rosenfeld et al. which requires a zeolitehaving a cesium and/or thallium content.

Typically, adsorbents used in separative processes contain thecrystalline material dispersed in an amorphous material or inorganicmatrix, having channels and cavities therein which enable liquid accessto the crystalline. Silica, alumina, or mixtures thereof are typical ofsuch inorganic matrix materials. The binder aids in forming oragglomerating the crystalline particles which otherwise would comprise afine powder. The adsorbent may thus be in the form of particles such asextrudates, aggregates, tablets, macrospheres or granules having adesired particle range, preferably from about 16 to about 60 mesh(Standard U.S. Mesh). Less water content in the adsorbent isadvantageous from the standpoint of less water contamination of theproduct.

The adsorbent may be employed in the form of a dense fixed bed which isalternatively contacted with a feed mixture and a desorbent material inwhich case the process will be only semicontinuous. In anotherembodiment, a set of two or more static beds of adsorbent may beemployed with appropriate valving so that a feed mixture can be passedthrough one or more adsorbent beds of a set while a desorbent materialcan be passed through one or more of the other beds in a set. The flowof a feed mixture and a desorbent material may be either up or downthrough an adsorbent in such beds. Any of the conventional apparatusemployed in static bed fluid-solid contacting may be used.

Moving bed or simulated moving bed flow systems, however, have a muchgreater separation efficiency than fixed bed systems and are thereforepreferred. In the moving bed or simulated moving bed processes, theretention and displacement operations are continuously taking placewhich allows both continuous production of an extract and a raffinatestream and the continual use of feed and displacement fluid streams. Onepreferred embodiment of this process utilizes what is known in the artas the simulated moving bed countercurrent flow system. In such asystem, it is the progressive movement of multiple liquid access pointsdown a molecular sieve chamber that simulates the upward movement ofmolecular sieve contained in the chamber. Reference can also be made toD. B. Broughton's U.S. Pat. No. 2,985,589, in which the operatingprinciples and sequence of such a flow system are described, and to apaper entitled, "Continuous Adsorptive Processing--A New SeparationTechnique" by D. B. Broughton presented at the 34th Annual Meeting ofthe Society of Chemical Engineers at Tokyo, Japan on Apr. 2, 1969, bothreferences incorporated herein by reference, for further explanation ofthe simulated moving bed countercurrent process flow scheme.

Another embodiment of a simulated moving bed flow system suitable foruse in the process of the present invention is the co-current highefficiency simulated moving bed process disclosed in U.S. Pat. No.4,402,832 to Gerhold, incorporated by reference herein in its entirety.

It is contemplated that at least a portion of the extract output streamwill pass into a separation means wherein at least a portion of thedesorbent material can be separated at separating conditions to producean extract product containing a reduced concentration of desorbentmaterial. Preferably, but not necessary to the operation of the process,at least a portion of the raffinate output stream will also be passed toa separation means wherein at least a portion of the desorbent materialcan be separated at separating conditions to produce a desorbent streamwhich can be reused in the process and a raffinate product containing areduced concentration of desorbent material. Typically the concentrationof desorbent material in the extract product and the raffinate productwill be less than about 5 vol.% and more preferably less than about 1vol.%. The separation means will typically be a fractionation column,the design and operation of which is well known to the separation art.

Although both liquid and vapor phase operations can be used in manyadsorptive separation processes, liquid-phase operation is preferred forthis process because of the lower temperature requirements and becauseof the higher yields of an extract product that can be obtained withliquid-phase operation over those obtained with vapor-phase operation.Adsorption conditions will include a temperature range of from about 20°C. to about 250° C. with about 100° C. to about 200° C. being morepreferred and a pressure sufficient to maintain liquid phase. Desorptionconditions will include the same range of temperatures and pressure asused for adsorption conditions.

The size of the units which can utilize the process of this inventioncan vary anywhere from those of pilot-plant scale (see for example U.S.Pat. No. 3,706,812) to those of commercial scale and can range in flowrates from as little as a few cc an hour up to many thousands of gallonsper hour.

The following examples are presented for illustration purposes and morespecifically are presented to illustrate the selectivity relationshipsthat make the process of the invention possible. Reference to specificcations, desorbent materials, feed mixtures and operating conditions isnot intended to unduly restrict the scope and spirit of the claimsattached hereto.

EXAMPLE I

In this experiment, three pulse tests were performed to evaluate theability of the present invention to separate diethylbenzene isomers. Theadsorbent used was a Y structured zeolite and a small portion ofamorphous binder material. The zeolite as supplied by the manufacturer,contained sodium cations at substantially all exchangeable cationicsites. Two of the tests used one of the desorbents within the scope ofthe present invention and, for comparison purposes, in one test adesorbent outside such scope was used. The adsorbent was dried toessentially complete dryness before it was utilized in the process.

The testing apparatus was the above described pulse test apparatus. Foreach pulse test, the column was maintained at a temperature of 150° C.and a pressure of 100 psig to maintain liquid-phase operations. Gaschromatographic analysis equipment was attached to the column effluentstream in order to determine the composition of the effluent material atgiven time intervals. The feed mixture employed for each test containedabout 5 vol.% each of the ethyltoluene isomers, 4 vol.% normal nonanewhich was used as a tracer and 81 vol.% desorbent material. Thedesorbent material in the first test was toluene (not in accordance withthe present invention), in the second test ethylbenzene and in the thirdtest m-xylene. In each test the desorbent comprised 30 vol.% of thearomatic material, with the remainder being n-C₇ paraffin. Theoperations taking place for each test were as follows. The desorbentmaterial was run continuously at a nominal liquid hourly space velocity(LHSV) of 1.0 which amounted to about 1.17 cc per minute feed rate ofdesorbent. At some convenient time interval the desorbent was stoppedand the feed mixture was run for a 10-minute interval at a rate of 1.0cc per minute. The desorbent stream was then resumed at 1 LHSV andcontinued to pass into the adsorbent column until all of the feedaromatics had been eluted from the column as determined by observing thechromatograph generated by the effluent material leaving the adsorptioncolumn. The sequence of operations usually takes about an hour. The10-minute pulse of feed and subsequent desorption may be repeated insequence as often as is desired. Selectivities derived from thechromatographic traces are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Desorbent:     Toluene  Ethylbenzene                                                                             m-Xylene                                   ______________________________________                                         ##STR2##       1.08 0.80                                                                              1.88 2.05  2.38 2.38                                 ______________________________________                                    

It is clear from Table 1 that only the aromatic desorbents of at leastC₈ weight when used with the sodium Y-type zeolite enable acceptableselectivities for the ortho-isomers.

EXAMPLE II

Pulse tests identical to those of Example I were run except that thefeedstream comprised 5 vol.% of each isomer of ethyltoluene, 4 vol.%normal nonane and 81 vol.% desorbent material. The resuls obtained withvarious desorbent materials are set forth in Table 2. In this example,half widths and retention volumes are also given for furtherillustration.

                  TABLE 2                                                         ______________________________________                                                 Ethyl- Meta    Ortho    Para                                                  Benzene                                                                              Xylene  Xylene   Xylene                                                                              P-DEB                                  ______________________________________                                        Desorbent                                                                     Half Widths cc                                                                n-Cg       10.1     10.4    9.9    10.9  10.0                                 p-EtTol    14.6     14.2    19.3   18.2  20.1                                 m-EtTol    14.3     14.5    17.8   18.1  20.6                                 o-EtTol    16.6     13.8    11.2   11.7  36.7                                 Retention Vol. cc                                                             p-EtTol    23.1     4.4     10.9   10.0  17.3                                 m-EtTol    21.0     5.1     13.0   11.5  17.6                                 o-EtTol    38.2     9.2     20.9   24.1  63.6                                 Selectivities                                                                 o/p        1.65     2.09    1.92   2.41  3.7                                  o/m        1.82     1.80    1.61   2.10  3.6                                  ______________________________________                                    

Again, it can be seen that in all embodiments of the present invention,the selectivities for ortho-ethyltoluene are satisfactory. Thepara-xylene is a particularly effective desorbent in view of the highselectivities and low retention volumes for the ortho-ethyltoluene, acombination which would ensure commercial applicability.

Further pulse tests employing ethylbenzene as a desorbent material overa temperature range from 125° C. to 175° C. indicated that the processtemperature does not significantly affect the separation.

EXAMPLE III

Pulse tests identical to those of Example I were run except that sodiumX or calcium Y-type zeolites were used with various desorbent materials.The results are as set forth in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Adsorbent                                                                              Na-X           Ca-Y                                                  Wt. % H.sub.2 O                                                                        ←                                                                             0    →                                                                           0    0  1.0                                                                              1.0                                        Desorbent                                                                              Toluene                                                                            Benzene                                                                            Et Benz.                                                                           Toluene                                                                            p-xyl                                                                            p-xyl                                                                            Benzene                                    __________________________________________________________________________    Half Widths, cc                                                               n-Cg     10.0 6.0  8.1  12.7 13.6                                                                             10.7                                                                             10.2                                       p-EtTol  22.7 17.8 18.3 25.4 31.2                                                                             21.4                                                                             21.4                                       m-EtTol  18.7 14.9 15.0 21.4 31.5                                                                             18.1                                                                             19.5                                       o-EtTol  6.5  21.5 15.8 21.8 34.5                                                                             17.4                                                                             17.3                                       Retention Vol., cc                                                            p-EtTol  17.7 13.8 21.3 17.8 15.0                                                                             19.1                                                                             17.0                                       m-EtTol  23.0 20.0 26.0 23.0 18.4                                                                             23.3                                                                             21.5                                       o-EtTol  32.1 36.0 34.5 42.2 45.5                                                                             39.9                                                                             39.1                                       Selectivities                                                                 o/p      1.8  2.6  1.6  2.4  3.0                                                                              2.1                                                                              2.3                                        o/m      1.4  1.8  1.3  1.8  2.5                                                                              1.7                                                                              1.8                                        __________________________________________________________________________

It is again apparent that satisfactory selectivities forortho-ethyltoluene are obtained for additional embodiments of thepresent invention, although the o/m selectivities for the Na-X/tolueneand Na-X/ethylbenzene are close to the borderline for practicalapplication.

It can also be seen from the data that when Ca-Y is employed, the watercontent thereof has an effect on selectivities. The optimum watercontent for that adsorbent is about 1.0 wt.%.

I claim as my invention:
 1. A process for separating the ortho-isomerfrom a feed mixture comprising at least two bi-alkyl substitutedmono-cyclic aromatic isomers, including the ortho-isomers, said isomershaving from 9 to about 18 carbon atoms per molecule, which processcomprises contacting, at adsorption conditions, said feed with anadsorbent comprising a crystalline aluminosilicate consistingessentially of a sodium Y-type zeolite which selectively adsorbs saidortho-isomer, removing said feed from said adsorbent, and recoveringsaid ortho-isomer by desorption at desorption conditions with adesorbent material comprising a C₈ or heavier monocyclic aromatic, saidfeed mixture and said desorbent material having boiling points of atleast 5° C. difference.
 2. The process of claim 1 wherein saidadsorption and desorption conditions include a temperature within therange of from about 20° C. to about 250° C. and a pressure sufficient tomaintain liquid phase.
 3. The process of claim 1 wherein said feedstockcomprises the isomers of ethyltoluene and said desorbent comprisesethylbenzene or a xylene.
 4. The process of claim 1 wherein saidfeedstock comprises the isomers of ethyltoluene and said desorbentcomprises p-diethylbenzene.
 5. The process of claim 1 wherein saidprocess is effected with a simulated moving bed flow system.
 6. Theprocess of claim 5 wherein said simulated moving bed flow system is ofthe countercurrent type.
 7. The process of claim 5 wherein saidsimulated moving bed flow system is of the co-current high efficiencytype.
 8. The process of claim 1 wherein said adsorbent includes anamorphous inorganic oxide matrix.
 9. A process for separating theortho-isomer from a feed mixture comprising at least two bi-alkylsubstituted monocyclic aromatic isomers, including the ortho-isomer,said isomers having from 9 to about 18 carbon atoms per molecule, whichprocess comprises contacting, at adsorption conditions, said feed withan adsorbent comprising a crystalline aluminosilicate consistingessentially of a sodium X or calcium Y-type zeolite which selectivelyadsorbs said ortho-isomer, removing said feed from said adsorbent, andrecovering said ortho-isomer by desorption at desorption conditions witha desorbent material comprising a C₈ or lighter monocyclic aromatic. 10.The process of claim 9 wherein said adsorption and desorption conditionsinclude a temperature within the range of from about 20° C. to about250° C. and a pressure sufficient to maintain liquid phase.
 11. Theprocess of claim 9 wherein said feedstock comprises the isomers ofethyltoluene.
 12. The process of claim 9 wherein said process iseffected with a simulated moving bed flow system.
 13. The process ofclaim 12 wherein said simulated moving bed flow system is of thecountercurrent type.
 14. The process of claim 12 wherein said simulatedmoving bed flow system is of the co-current high efficiency type. 15.The process of claim 9 wherein said adsorbent includes an amorphousinorganic oxide matrix.
 16. The process of claim 9 wherein saidadsorbent contains about 1 wt.% water.